Research on Maintenance Management and Maintenance of CNC Machining Centers
Abstract: This paper elaborates in detail on the importance of maintenance management and maintenance of CNC machining centers, and deeply analyzes the same contents in maintenance management between CNC machining centers and ordinary machine tools, including the system of assigning specific personnel to operate, maintain and hold certain positions, job training, inspection and maintenance systems, etc. Meanwhile, it emphasizes the unique contents in the maintenance management of CNC machining centers, such as the rational selection of maintenance methods, the establishment of professional maintenance organizations and maintenance cooperation networks, and comprehensive inspection management. It also provides a detailed description of the specific maintenance points on a daily, semi-annual, annual and irregular basis, aiming to provide comprehensive guidance on maintenance management and maintenance for the efficient and stable operation of CNC machining centers.
I. Introduction
As the key equipment in modern manufacturing industry, CNC machining centers integrate multidisciplinary technologies such as machinery, electricity, hydraulics, and numerical control, and possess remarkable characteristics such as high precision, high efficiency, and a high degree of automation. They are widely used in numerous fields like aerospace, automobile manufacturing, and mold processing, and play a decisive role in product quality and production efficiency. However, CNC machining centers have complex structures and high technological content. Once a malfunction occurs, it will not only lead to production stoppage and cause huge economic losses but also may affect product quality and corporate reputation. Therefore, scientific and effective maintenance management and maintenance are of vital importance for CNC machining centers.
II. The Same Contents in Maintenance Management between CNC Machining Centers and Ordinary Machine Tools
(I) System of Assigning Specific Personnel to Operate, Maintain and Hold Certain Positions
During the use of equipment, the system of assigning specific personnel to operate, maintain and hold certain positions must be strictly adhered to. This system clarifies the operators, maintenance personnel of each piece of equipment and their corresponding job positions and scope of responsibilities. By assigning the responsibilities for the use and maintenance of equipment to specific individuals, the familiarity and sense of responsibility of operators and maintenance personnel towards the equipment can be enhanced. Operators can better grasp the operational characteristics and subtle changes of the equipment during the long-term use of the same equipment and promptly detect abnormal situations. Maintenance personnel can also have a deeper understanding of the structure and performance of the equipment, conduct maintenance and troubleshooting more accurately, thereby improving the utilization efficiency and stability of the equipment and reducing problems such as equipment misoperation and inadequate maintenance caused by frequent personnel changes or unclear responsibilities.
(II) Job Training and Prohibition of Unauthorized Operation
Conducting comprehensive job training is the basis for ensuring the normal operation of equipment. Operators and maintenance personnel of both CNC machining centers and ordinary machine tools need to receive systematic training, including equipment operation specifications, safety precautions, basic maintenance knowledge, etc. Unauthorized operation is strictly prohibited. Only personnel who have received professional training and passed the assessment are allowed to operate the equipment. Unauthorized personnel, due to the lack of necessary equipment operation knowledge and skills, are very likely to cause equipment malfunctions or even safety accidents due to misoperation during the operation process. For example, those who are not familiar with the functions of the control panel of the machine tool may set processing parameters incorrectly, resulting in collisions between cutting tools and workpieces, damage to key components of the equipment, affecting the precision and service life of the equipment, and also posing a threat to the safety of operators themselves.
(III) Equipment Inspection and Regular, Graded Maintenance Systems
Strict implementation of the equipment inspection system is an important means to promptly detect potential problems of the equipment. Both CNC machining centers and ordinary machine tools need to conduct comprehensive inspections on the equipment according to the specified inspection cycles and contents. The inspection contents cover all aspects of the equipment, such as the mechanical components, electrical systems, and hydraulic systems, including checking the lubrication status of the machine tool guide rails, the connection tightness of transmission components, and whether the connections of electrical circuits are loose, etc. Through regular inspections, abnormal signs can be detected in time before equipment malfunctions occur, and corresponding measures can be taken for repair to avoid the expansion of malfunctions.
The regular and graded maintenance systems are formulated from the perspective of the overall maintenance of the equipment. Based on the usage time and operating conditions of the equipment, different levels of maintenance plans are developed. Regular maintenance includes work such as cleaning, lubricating, adjusting, and tightening the equipment to maintain its good operating condition. Graded maintenance determines different levels of maintenance standards and requirements according to the importance and complexity of the equipment to ensure that key equipment receives more refined and comprehensive maintenance. For example, for the spindle box of an ordinary machine tool, during regular maintenance, it is necessary to check the oil quality and quantity of the lubricating oil and clean the filters. During graded maintenance, it may be necessary to check and adjust the preload of the spindle bearings to ensure the rotational precision and stability of the spindle.
(IV) Maintenance Records and Archive Management
Implementing the job assignment card system for maintenance personnel and carefully recording detailed information such as the phenomena, causes, and maintenance processes of malfunctions and establishing complete maintenance archives are of great significance for the long-term management of equipment. Maintenance records can provide valuable reference materials for subsequent equipment maintenance and troubleshooting. When similar malfunctions occur again in the equipment, maintenance personnel can quickly understand the previous malfunction handling methods and the information on replaced parts by referring to the maintenance archives, thereby improving maintenance efficiency and reducing maintenance time. Meanwhile, maintenance archives also help analyze the malfunction patterns and reliability of the equipment and provide a basis for formulating reasonable equipment renewal and improvement plans. For example, through the analysis of the maintenance archives of a certain machine tool, it is found that a certain component in its electrical system frequently malfunctions after running for a certain period of time. Then, it can be considered to replace this component in advance or optimize the design of the electrical system to improve the reliability of the equipment.
(V) Maintenance Cooperation Network and Expert Diagnosis System
Establishing a maintenance cooperation network and carrying out the work of the expert diagnosis system have a positive effect on improving the equipment maintenance level and solving complex malfunctions. Within an enterprise, different maintenance personnel have different professional skills and experiences. Through the maintenance cooperation network, technical exchanges and resource sharing can be realized. When encountering difficult malfunctions, they can pool their wisdom and jointly explore solutions. The expert diagnosis system makes intelligent diagnoses of equipment malfunctions with the help of computer technology and the knowledge base of expert experience. For example, by inputting the common malfunction phenomena, causes, and solutions of CNC machining centers into the expert diagnosis system, when the equipment malfunctions, the system can give possible malfunction causes and maintenance suggestions according to the input malfunction information, providing powerful technical support for maintenance personnel. Especially for some maintenance personnel with insufficient experience, it can help them locate and solve malfunctions more quickly.
III. Contents to be Emphasized in the Maintenance Management of CNC Machining Centers
(I) Rational Selection of Maintenance Methods
The maintenance methods of CNC machining centers include corrective maintenance, preventive maintenance, corrective and preventive maintenance, predictive or condition-based maintenance, and maintenance prevention, etc. The rational selection of maintenance methods needs to consider various factors comprehensively. Corrective maintenance means conducting maintenance after the equipment malfunctions. This method is applicable to some non-critical equipment or situations where the consequences of malfunctions are minor and the maintenance costs are low. For example, when some auxiliary lighting equipment or non-critical cooling fans of a CNC machining center malfunction, the corrective maintenance method can be adopted. They can be replaced in time after being damaged, and it will not have a significant impact on production.
Preventive maintenance is to conduct maintenance on the equipment according to the predetermined cycle and contents to prevent malfunctions from occurring. This method is applicable to situations where equipment malfunctions have obvious time periodicity or wear patterns. For example, for the spindle bearings of a CNC machining center, they can be replaced or maintained regularly according to their service life and running time, which can effectively prevent the decline in spindle precision and malfunctions caused by bearing wear.
Corrective and preventive maintenance is to improve the equipment during the maintenance process to enhance its performance or reliability. For example, when it is found that there are unreasonable aspects in the structural design of a CNC machining center, resulting in unstable processing precision or frequent malfunctions, the structure can be optimized and renovated during maintenance to improve the overall performance of the equipment.
Predictive or condition-based maintenance is to monitor the operating status of the equipment in real-time through advanced monitoring technologies, predict possible malfunctions of the equipment according to the monitoring data, and conduct maintenance before malfunctions occur. This method is particularly important for the key components and systems of CNC machining centers. For example, by using technologies such as vibration analysis, temperature monitoring, and oil analysis to monitor the spindle system, when it is found that the vibration value increases abnormally or the oil temperature is too high, the spindle can be inspected and maintained in time to avoid serious damage to the spindle and ensure the high-precision operation of the machining center. Maintenance prevention considers the maintainability of the equipment from the design and manufacturing stages to make the equipment easier to maintain in the subsequent use process. When selecting a CNC machining center, attention should be paid to its maintenance prevention design, such as the modular design of components and structures that are easy to disassemble and install. When evaluating maintenance methods, comprehensive assessments need to be made from aspects such as repair costs, production stoppage losses, maintenance organization work, and repair effects. For example, for a CNC machining center with high value and a busy production task, although the investment in monitoring equipment and technologies for predictive maintenance is relatively high, compared with the long-term production stoppage losses caused by sudden equipment malfunctions, this investment is worthwhile. It can effectively reduce the downtime of the equipment, improve production efficiency, and ensure the product delivery cycle.
(II) Establishment of Professional Maintenance Organizations and Maintenance Cooperation Networks
Due to the complexity and advanced technology of CNC machining centers, establishing professional maintenance organizations is the key to ensuring their normal operation. Professional maintenance organizations should be equipped with maintenance personnel who have professional knowledge and skills in multiple aspects such as machinery, electricity, and numerical control. These personnel should not only be familiar with the hardware structure of CNC machining centers but also master the programming, debugging, and malfunction diagnosis technologies of their numerical control systems. The internal maintenance organizations should have complete maintenance tools and testing equipment, such as high-precision measuring tools, electrical testing instruments, and numerical control system diagnostic instruments, to meet the maintenance needs of different types of malfunctions.
Meanwhile, establishing a maintenance cooperation network can further enhance the maintenance ability and resource utilization efficiency. The maintenance cooperation network can cover equipment manufacturers, professional maintenance service companies, and the maintenance departments of other enterprises in the industry. By establishing a close cooperative relationship with equipment manufacturers, it is possible to obtain technical materials, maintenance manuals, and the latest software upgrade information of the equipment in a timely manner. In the event of major malfunctions or difficult problems, remote guidance or on-site support from the technical experts of the manufacturers can be obtained. By cooperating with professional maintenance service companies, when the enterprise’s own maintenance strength is insufficient, external professional strength can be borrowed to quickly solve equipment malfunctions. The maintenance cooperation among enterprises in the industry can realize the sharing of maintenance experience and resources. For example, when an enterprise accumulates valuable experience in repairing a special malfunction of a certain model of CNC machining center, this experience can be shared with other enterprises through the maintenance cooperation network, avoiding other enterprises from repeating the exploration when encountering the same problem and improving the maintenance level of the entire industry.
(III) Inspection Management
The inspection management of CNC machining centers conducts comprehensive management on the equipment in terms of fixed points, fixed times, fixed standards, fixed items, fixed personnel, fixed methods, inspection, recording, handling, and analysis according to relevant documents.
Fixed points refer to determining the parts of the equipment that need to be inspected, such as the guide rails, lead screws, spindles, and electrical control cabinets of the machine tool, which are key parts. These parts are prone to problems such as wear, looseness, and overheating during the operation of the equipment. Abnormalities can be detected in time through fixed-point inspections. Fixed standards are to set normal standard values or ranges for each inspection point. For example, the rotational precision of the spindle, the straightness of the guide rails, and the pressure range of the hydraulic system. During inspection, the actual measured values are compared with the standard values to judge whether the equipment is normal. Fixed times are to clarify the inspection cycle of each inspection item, which is determined according to factors such as the running time, work intensity, and wear patterns of the components, such as inspection items with different cycles like daily, weekly, and monthly. Fixed items are to stipulate specific inspection contents, such as checking the rotational speed stability of the spindle, the lubrication status of the lead screw, and the grounding reliability of the electrical system. Fixed personnel are to assign specific responsible persons for each inspection item to ensure the implementation of the inspection work. Fixed methods are to determine the inspection methods, including the use of detection tools, instruments, and the operation steps of inspection, such as using a micrometer to measure the straightness of the guide rails and using an infrared thermometer to detect the temperature of the spindle.
During the inspection process, inspection personnel conduct inspections on the equipment according to the specified methods and cycles and make detailed records. The record contents include information such as inspection time, inspection parts, measured values, and whether they are normal. The handling link is to take corresponding measures in a timely manner for the problems found during inspection, such as adjusting, tightening, lubricating, and replacing parts. For some minor abnormalities, they can be handled immediately on the spot. For more serious problems, a maintenance plan needs to be formulated and professional maintenance personnel are arranged to conduct maintenance. Analysis is an important part of inspection management. By analyzing the inspection records within a certain period of time, the operating status and malfunction patterns of the equipment are summarized. For example, if it is found that the frequency of abnormal situations in a certain part gradually increases, it is necessary to conduct in-depth analysis of the reasons. It may be due to increased wear of components or changes in the working environment of the equipment. Then, preventive measures can be taken in advance, such as adjusting equipment parameters, improving the working environment, or preparing to replace parts in advance.
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Daily Inspection
Daily inspection is mainly carried out by machine tool operators. It is the inspection of general components of the machine tool and the handling and inspection of malfunctions that occur during the operation of the machine tool. For example, it is necessary to check the oil level gauge and oil quantity of the guide rail lubricating oil tank every day to ensure that lubricating oil is added in time, so that the lubricating pump can start and stop regularly to ensure good lubrication of the guide rails and reduce wear. Meanwhile, it is necessary to remove chips and dirt on the guide rail surfaces of the XYZ axes, check whether the lubricating oil is sufficient, and check whether there are scratches or damages on the guide rail surfaces. If scratches are found, repair measures should be taken in time to prevent them from further deteriorating and affecting the precision of the machine tool. Check whether the pressure of the compressed air source is within the normal range, clean the automatic water separation filter and automatic air dryer of the air source, and promptly remove the water filtered out by the water separation filter to ensure the normal operation of the automatic air dryer and provide a clean and dry air source for the pneumatic system of the machine tool to prevent pneumatic component malfunctions caused by air source problems. It is also necessary to check the oil levels of the gas-liquid converter and booster. When the oil level is insufficient, replenish the oil in time. Pay attention to whether the oil quantity in the spindle lubricating constant temperature oil tank is sufficient and adjust the temperature range to provide stable lubrication and a suitable working temperature for the spindle to ensure the high-precision operation of the spindle. For the hydraulic system of the machine tool, check whether there are abnormal noises in the oil tank and hydraulic pump, whether the pressure gauge indication is normal, whether there are leaks in the pipelines and joints, and whether the working oil level is normal to ensure the stable operation of the hydraulic system, because the hydraulic system plays a key role in actions such as clamping and tool changing of the machine tool. Check whether the balance pressure indication of the hydraulic balance system is normal and observe whether the balance valve works normally when the machine tool moves rapidly to prevent the imbalance of the moving parts of the machine tool caused by the malfunction of the balance system, which may affect the processing precision and equipment safety. For the input and output units of the CNC, keep the photoelectric reader clean, ensure good lubrication of the mechanical structure, and ensure the normal data transmission between the numerical control system and external equipment. In addition, check the heat dissipation and ventilation devices of various electrical cabinets to ensure that the cooling fans of each electrical cabinet work normally and that the air duct filter screens are not blocked to prevent the damage of electrical components caused by excessive temperature inside the electrical cabinets. Finally, check various protective devices, such as guide rails and various protective covers of the machine tool, to ensure that they are not loose to ensure the operation safety of the machine tool and prevent foreign objects such as chips and cooling liquid from entering the inside of the machine tool and damaging the equipment. -
Full-time Inspection
Full-time inspection is carried out by full-time maintenance personnel. It mainly focuses on conducting key inspections on the key parts and important components of the machine tool according to the cycle and carrying out equipment status monitoring and malfunction diagnosis. Full-time maintenance personnel need to formulate detailed inspection plans and conduct regular inspections on key components such as ball screws according to the plans. For example, clean the old grease of the ball screw and apply new grease every six months to ensure the transmission precision and smoothness of the screw. For the hydraulic oil circuit, clean the relief valve, pressure reducing valve, oil filter, and the bottom of the oil tank every six months, and replace or filter the hydraulic oil to prevent hydraulic system malfunctions caused by oil contamination. Check and replace the carbon brushes of the DC servo motor every year, check the surface of the commutator, blow off the carbon powder, remove burrs, replace the carbon brushes that are too short, and use them after running-in to ensure the normal operation and good speed regulation performance of the motor. Clean the lubricating hydraulic pump and oil filter, clean the bottom of the pool, and replace the oil filter to ensure the cleanness and normal liquid supply of the lubricating system. Full-time maintenance personnel also need to use advanced detection equipment and technologies to monitor the state of the machine tool. For example, use vibration analysis instruments to monitor the spindle system, analyze the vibration spectrum to judge the operating state and potential malfunctions of the spindle. Use oil analysis technology to detect the oil in the hydraulic system and spindle lubricating system, and judge the wear condition of the equipment and the contamination degree of the oil according to indicators such as the content of metal particles and viscosity changes in the oil to detect potential malfunction hazards in advance and formulate corresponding maintenance strategies. Meanwhile, make diagnosis records according to the inspection and monitoring results, deeply analyze the maintenance results, and put forward suggestions for improving equipment maintenance management, such as optimizing the inspection cycle, improving the lubrication method, and increasing protective measures to continuously improve the reliability and stability of the equipment. -
Other Regular and Irregular Maintenance Points
In addition to daily and full-time inspections, CNC machining centers also have some maintenance points that are carried out on a semi-annual, annual,